There are a wide range of methods and arrangements for setting up and operating pulse radar sensors, known for a long time from [1], [2] and [3] inter alia. Pulse radar sensors are used as fill level sensors in industrial metering technology, parking aids or close-range sensors in motor vehicles to prevent collision, to map surroundings and for the navigation of autonomous vehicles and transport systems, e.g. robots and conveyor units. Generally pulse radar sensors operate in the areas of application listed at center frequencies of approximately 1 GHz to 100 GHz with typical pulse lengths of 200 ps to 2 ns. Such sensors have been referred to for some time as ultrawideband (UWB) radar due to their large measurable bandwidth. Almost all pulse radar sensors have in common the fact that their measurement signals have such a large bandwidth that the signals cannot be received directly and processed using standard technologies. Therefore almost all known systems use so-called sequential sampling systems. With the sequential sampling principle, which is known from former digital sampling oscilloscopes, the measurement signal is sampled sequentially over a plurality of measurement cycles by displacing the sampling times sequentially.
Solutions for pulse radar using circuit technology are for example known from the above-mentioned prior art. The prior art describes a transmit pulse with a defined repetition frequency CLK-Tx (Clock Transmission), which is transmitted and the reflected receive signal is sampled with a sampling system with a repetition frequency CLK-Rx (Clock Reception). If the frequencies of the transmit sequence differ slightly from those of the sampling sequence, the phases of the two sequences move slowly towards each other. This slow relative displacement of the sampling time towards the transmit time brings about a sequential sampling process.
FIG. 1 shows a known embodiment of a pulse radar operating in the manner described above. In a transmit unit a transmit clock generator AT generates a clock frequency CLK-Tx, with which a pulse generator BT generates short voltage pulses cyclically. A high-frequency oscillator CT is then activated with these short pulses and generates high-frequency oscillations during the activation period, which are transmitted as transmit signals DT via the antenna ET. An identical pulse generator chain is set up in a receive branch or in a receive unit with the corresponding elements AR, BR and CR. The pulse signal from the oscillator CR is passed to a mixer M, which therefore also functions as the sampling system, as the mixer is also supplied with the receive signal DR from the other side. The signal elements of the transmit signal D of the transmit branch reflected off an object O and returned to the receive antenna ER as a receive signal DR are mixed by the mixer M with the signal from CR of a low-frequency base band. The sampling pulse sequence thus generated is smoothed by a bandpass filter BPF and thus ultimately produces the measurement signal LFS (generally Low Frequency Signal).
To achieve a good signal to noise ratio (SNR) of the measurement signal it is crucial that the oscillators CT and CR have a deterministic, i.e. not a stochastic, phase relationship to each other over all the pulses in a sequence. Such a deterministic relationship of the pulses generated by CT and CR is not simply achieved, as CT and CR operate independently of each other. A deterministic relationship results however when the pulse signals activating pulse generators BT and BR are such that they generate harmonic waves, which are in the frequency band of the high-frequency oscillators CT and CR. The harmonic waves cause the oscillators CT and CR not to oscillate stochastically on activation but to be activated coherently in respect of the harmonic waves of the signals BT and BR. As the signals and harmonic waves from the pulse generators BT and BR are always the same with each activation process, CT and CR respectively always oscillate with a characteristic fixed initial phase, so that their signals have a deterministic phase and time relationship to each other, predetermined by the transmit signal sequence and the sampling signal sequence.
Methods for ensuring the deterministic relationship of the transmit and sampling pulses are known from the prior art, in which a single continuously operating fixed-frequency oscillator is generally used, from which the required pulses are derived using switches. It is also known that a common antenna can be used for transmitting and receiving instead of separate antennae such as ET and ER, the transmit and receive signals being separated for example by means of a route matrix switch.
However in many applications it is preferable not only for distances to be measured one-dimensionally using a radar sensor but also for there to be the option of mapping object scenarios in a multi-dimensional manner. For three-dimensional scenario mapping for example and thereby accurate determination of distance from the object, the sensors and/or their measurement directions are either moved and measurements are taken one after the other at different sites or in different directions and/or systems are used with a plurality of spatially distributed sensors. Such systems are for example known from [4] as “multistatic sensor systems”. With multistatic sensor systems with a plurality of spatially distributed transmitters and receivers it is advantageous if one of the transmitters respectively transmits a signal, which is reflected off the object scenario and then detected by all the receivers. Such arrangements and their mode of operation however have the disadvantage that a large outlay is generally required to couple spatially distributed transmit and receive branches such that the phases of their high-frequency signal sources have a deterministic relationship to each other.
As described above, a deterministic phase relationship is a basic precondition for achieving a good signal to noise ratio. Deriving high-frequency signals from a common source and distributing them spatially by means of high-frequency lines is however disadvantageous for commercial applications in particular, as very high costs are incurred and signal attenuation and dispersion of the transmitted signals result. Phase control circuits for coupling a plurality of oscillators are generally excluded for similar reasons.